dt-bindings: display: mediatek: dp: Add compatible for MediaTek MT8188

Add dt-binding documentation of dp-tx for MediaTek MT8188 SoC.

Signed-off-by: Shuijing Li <shuijing.li@mediatek.com>
Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230822024155.26670-2-shuijing.li@mediatek.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>